1. Field of the Invention
The present invention relates to an exposure apparatus for exposing a substrate with a pattern image projected by a projection optical system while filling, with a liquid, at least a part of a space between the projection optical system and the substrate. The present invention also relates to a liquid-removing apparatus to be used for the exposure apparatus, and a method for producing a device based on the use of the exposure apparatus.
2. Description of the Related Art
Semiconductor devices and liquid crystal display devices are produced by the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used becomes shorter, the resolution of the projection optical system is higher. As the numerical aperture of the projection optical system becomes larger, the resolution of the projection optical system is higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to utilize the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases) so that the resolution is improved and the depth of focus is magnified about n times.
When the substrate is subjected to the exposure process by using the liquid immersion method, the liquid remains in some cases on the surface of the substrate after the exposure process. If the substrate is transported in a state in which the remaining liquid adheres to the substrate, then the liquid falls from the substrate during the transport, and inconveniences arise, for example, such that respective apparatuses and members, which are disposed around the transport passage, become rusty due to the fallen liquid, and/or the cleanness cannot be maintained in the environment in which the exposure apparatus is arranged. In other cases, the environmental change (humidity change) is caused by the fallen liquid around the exposure apparatus. If the humidity change is caused, the following problems arise. That is, for example, any fluctuation arises in the air on the optical path of the optical interferometer which is to be used to measure the position of the stage. The position of the stage is not measured accurately, and it is impossible to obtain any desired pattern transfer accuracy. Further, for example, if the development process is executed in a state in which the liquid adheres to the substrate after the exposure process, it is feared that any device having desired performance cannot be produced.